Added Tox file to verify tests.

Tox file added to verify that tests pass on Python 2.6, 2.7, and 3.3 and on Django 1.4 / 1.5. Copied verbatim from @jacobian https://github.com/jacobian/django-markupfield/blob/1c9b3fd7665417f977e41063666ff21a5dda8596/tox.ini